UBC Theses and Dissertations
Molecular simulation of nucleation and dissolution of alkali halides Lanaro, Gabriele
The process of crystal nucleation, despite being so fundamental and ubiquitous in industrial and natural processes, is still not fully understood because of its stochastic nature, and the high spatial and temporal resolution needed to observe it through experiments. This thesis investigates several aspects of nucleation through the use of molecular dynamics, a computational technique that is able to simulate systems up to ~10¹² atoms (as of today's computational power). The projects in this thesis focus on the nucleation from aqueous solution of alkali halide salts, with supplementary studies on the related processes of dissolution in water, and crystallization from the melt. The mechanism of NaCl nucleation from solution is examined in Chapter 3 by direct simulation. The NaCl supersaturated solution was found to contain many small ionic clusters that continuously form and disappear from solution until one (or more) of them nucleates and grows irreversibly. An original method was developed to detect and follow clusters in time, producing results useful in the study of their characteristics and lifetimes. Most importantly, it was found that the lifetime of transient clusters is about ~1 ns, and that both the cluster lifetime and nucleation probability are significantly higher if the cluster is more geometrically ordered. The dissolution of NaCl crystals was also investigated. The process was found to happen in stages, is characterized by an activation barrier, and can be described by a simple rate law. The crystal nucleation of LiF from supersaturated solution was observed, in our simulations, only at high pressure and temperature. The growth rate for an already nucleated crystal was found to have a temperature dependence that follows the Arrhenius law, and further evidence suggests that the reason for such behavior is the high activation energy required to dehydrate the ions. The crystallization from the melt of the Joung-Cheatham and Tosi-Fumi models for lithium halides was also investigated. We found that, for the Tosi-Fumi model, all lithium halides crystallize as wurtzite. For the Joung-Cheatham model, LiF and LiCl crystallize as rock salt, while LiBr and LiI crystallize as wurtzite.
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